1. Technical Field
The present invention relates to a zinc-nickel plating bath.
2. Description of the Prior Art
JP 1298192A discloses an alkaline cyanide-free zinc nickel plating solution having a pH greater than 13. The plating solution contains 5-20 g/L Zn2+, 0.4-4.0 g/L Ni2+, 0.04 mol/L of one or more amine chelating agents selected from N-aminoethylethanolamine, ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine, and an effective amount of a brightener, such as a quaternary pyridine compound.
U.S. Pat. No. 5,405,523 discloses a zinc alloy electroplating bath comprising a ureylene quaternary ammonium polymer as a brightening agent. The bath can also contain a supplemental brightener. One suitable supplemental brightener which is listed is n-benzyl nicotinic acid (sodium salt). Nickel is listed as a metal which can be alloyed with the zinc.
U.S. Pat. No. 4,889,602 discloses an alkaline zinc-nickel electroplating bath which comprises an aliphatic amine or polymer of an aliphatic amine in combination with an hydroxyaliphatic carboxylic acid.
U.S. Pat. Nos. 4,071,418 and 4,071,419 disclose the combination of amines with substituted pyridines such as nicotinic acid or nicotinamide for a zinc bath.
U.S. Pat. No. 5,417,840 discloses an alkaline zinc-nickel plating bath comprising a polyamine such as polyethyleneimine in combination with an aromatic heterocyclic nitrogen containing compound such as a sulfo-betaine, e.g., pyridinium-N-propane-3-sulfonic acid; or a pyridinium chloride such as N-carboxymethyl pyridinium chloride.
U.S. Pat. Nos. 4,730,022 and 4,210,500 disclose the use of an aromatic carboxyl compound such as 1-benzyl-pyridinium-3-carboxylate or 3-pyridine carboxylic acid (nicotinic acid) as a supplementary brightener in an alkaline zinc bath. The carboxyl compounds are used in combination with a primary brightener, such as the reaction product of a polyamine and a sulfonate.
The present invention resides in an alkaline zinc-nickel electroplating bath that comprises zinc ions, nickel ions, a primary brightener, which is an N-methylpyridinium compound substituted at the 3-position of the pyridine ring with a carboxylate group or a group which is hydrolyzable to a carboxylate group, and a secondary brightener, which is an aliphatic amine. Examples of hydrolyzable groups at the 3-position are carboxamides, carboxylate esters, and nitrites. An example of a preferred primary brightener is N-methylpyridinium-3-carboxylate (or a salt thereof, e.g., sodium or potassium salt thereof).
A preferred aliphatic amine is a polyethyleneimine.
Preferably, the electroplating bath has a pH more than about 13.
The electroplating bath of the present invention is an aqueous alkaline bath having a pH that is preferably above about 13, and more preferably, above about 14. The bath contains an inorganic alkaline component in an effective amount to achieve this pH. Amounts of from about 50 grams per liter to about 200 grams per liter, based on the electroplating bath, of the alkaline component can be used. Examples of suitable alkaline components are alkali metal derivatives such as sodium hydroxide and potassium hydroxide.
The electroplating bath contains a controlled amount of zinc ions and a controlled amount of nickel ions. The source for the zinc ions for the electroplating bath can be any zinc compound which is soluble in an alkaline aqueous medium. Examples of zinc compounds which can be added to the electroplating bath are zinc oxide, or a soluble salt such as zinc sulfate, zinc carbonate, zinc sulfamate, and zinc acetate. The concentration of zinc ions in the electroplating bath is from about 1 to about 100 grams per liter, preferably about 4 to about 50 grams per liter (about 4,000 to about 50,000 ppm). At a pH above about 13, the predominant zinc species in the bath is the zincate ion.
The source for the nickel ions for the electroplating bath can be any nickel compound which can be made soluble in an aqueous alkaline solution. Examples of suitable nickel compounds are an inorganic or organic acid salt of nickel, such as nickel sulfate, nickel carbonate, nickel acetate, nickel sulfamate and nickel formate. The concentration of nickel ions in the electroplating bath can be from about 0.1 to about 10 grams per liter, (about 100 to about 10,000 ppm), more preferably in the range from about 0.1 gram per liter to about 3 grams per liter (about 100 ppm to about 3,000 ppm).
The electroplating bath of the present invention comprises a combination of brighteners.
The primary brightener in the bath is an N-methyl pyridinium compound that has the following structural formula: 
in which R is a carboxylate group or a group that can undergo alkaline hydrolysis to a carboxylate group. For purposes of the present application, the term xe2x80x9cN-methylpyridinium compoundxe2x80x9d is also meant to include salts of the N-methylpyridinium compound such as sodium or potassium salts of the N-methylpyridinium compound.
Examples of groups that are hydrolyzable to a carboxylate group are carboxamides, carboxylate esters and nitrites. Carboxylate esters of N-methylnicotinic acid which are useful as esters react with hydroxyl ions to form the carboxylate group. Nitriles hydrolyze to amides and then to carboxylates and are also useful.
A preferred primary brightener in the electroplating bath of the present invention is N-methylpyridinium-3-carboxylate (or salt thereof). This compound is commonly known as trigonelline or N-methylnicotinate and has a carboxylate group at the 3-position of the pyridine ring as shown below: 
One method of preparing N-methylpyridinium-3-carboxylate involves reacting nicotinic acid (C5H4NCOOH) with sodium hydroxide in an aqueous solution to produce sodium nicotinate. The sodium nicotinate is reacted with methyl iodide to produce the N-methylpyridinium-3-carboxylate. The reaction mixture is then diluted to about a 10% by weight solution. Other methods of preparing N-methylpyridinium-3-carboxylate are well known in the art and can be used to prepare the N-methylpyridinium-3-carboxylate of the present invention.
The primary brightener in the electroplating bath of the present invention is present preferably in an amount in the range of about 0.01 to about 1 gram per liter (about 10 ppm to about 1,000 ppm).
The electroplating bath of the present invention also comprises a secondary brightener. The secondary brightener in the bath is an aliphatic amine. A preferred aliphatic amine is an aliphatic polyamine. The molecular weight of the aliphatic polyamine is not critical and can be the molecular weight of any aliphatic polyamine that is commercially available.
Preferred aliphatic amines, which can be used as a secondary brightener, are aliphatic polyamines derived from ethyleneimine (aziridine). Preferred aliphatic polyamines derived from ethyleneimine are tetraethylenepentamine (TEPA) and diethylenetetramine. Another preferred aliphatic polyamine derived from ethyleneimine is a polyethyleneimine marketed by BASF Corporation of Parsippany, N.J. under the trademark POLYMIN G-35. POLYMIN G-35 is a polyethyleneimine that has the general formula (C2H5N)n and a molecular weight of about 3500.
In the electroplating bath of the present invention, the aliphatic amine performs a dual function. In addition to functioning as a secondary brightener, the aliphatic amine also serves as a complexing agent for the nickel ions. Nickel is non-amphoteric, and at a high pH, forms nickel hydroxide, which is insoluble and precipitates out of solution. To prevent precipitation of nickel hydroxide, a complexing agent is added to the bath, which complexes the nickel ions and prevents the formation of the nickel hydroxide. In a preferred embodiment of the present invention, the complexing agent is the aliphatic amine of the present invention.
With the use of the brighteners of the present invention in the electroplating bath of the present invention, not only is a bright deposit obtained, but also one having a relatively high nickel content, at least about 4% by weight, and preferably above about 10% by weight.
A primary application for the zinc-nickel electroplating bath of the present invention is to provide a base plate for automotive components exposed to severe corrosion. Typically, the components are treated with a chromate conversion coating. It is known that a relatively high nickel content in the zinc-nickel plate, preferably in the range of about 10% to about 16% by weight nickel, provides improved resistance to corrosion and a better appearance with such a coating.
Using the combination of N-methylpyridinium compound of the present invention with an aliphatic amine resulted in bright deposits that were resistant to blistering and that had nickel in the desired concentration range above about 4% by weight, and preferably above about 10% by weight, e.g., 12% nickel and 88% zinc.
The amount of aliphatic amine that is used in the zinc-nickel electroplating bath is within the range of about 2.5 to about 60 grams per liter of bath (2,500 to 60,000 ppm).
The bath of the present invention can also comprise a hydroxyl amine such as triethanolamine as an auxiliary brightener.
The electroplating baths of the present invention are used at conventional current densities, about 1 to about 100 amperes per square foot as determined by Hull Cell evaluation. Bright deposits are obtained at conventional temperatures, for instance about 20xc2x0 C. to about 40xc2x0 C.
The following example illustrates the present invention.